Spec Scripts vs Shooting Scripts
Beginners are often told to read produced screenplays in order to learn the basic formatting and approach to writing for movies. In theory, this is a great piece of advice to get you started, but there are a few problems inherent in the approach.
Firstly, most of the screenplays that are legally available on the internet (or included on Blu-Rays or DVDs as special features) are the final shooting drafts. While this makes perfect sense from the perspective of the studios or writers who released those scripts, it also presents a major issue that confuses beginners:
Shooting scripts are not the original spec scripts that sold.
From a subjective standpoint, shooting scripts are sometimes far more polished... or, in some cases, just a giant mess that resulted from the many different contributors who re-wrote it during the production process - from directors and producers to actors and other writers.
While the actual movie may have been made from that final shooting draft, it's often a very different beast from the original script that turned heads and triggered the sale.
So what are the differences, exactly?
Well, the subjective differences could be anything but the most obvious and objective differences are scene numbers and revision marks.
A shooting script is intended for practical use on set. A spec script ("spec" meaning it's written "on speculation") is intended for sale and will likely be revised hundreds of times before it's used on a movie set.
Scene numbers appear to the left and right of each slugline (scene heading) in a shooting script... they are not used in spec scripts.
Revision marks appear as asterisks on the right edge of the page, signifying that something was changed in the most recent draft of a shooting script. Development execs may sometimes want these later on if you're working on a re-write for them, but the draft of your spec script that you send out for consideration should never include them.
If you've got what it takes to write a sellable script, the subjective differences that separate a marketable spec from an un-marketable one should take care of themselves. Just write your best, re-write, and keep growing your own experience as well as the quality of the script. After that, you can't do anything about subjectivity... but you can easily stack the odds in your favor when it comes to simple things like scene numbers and revision marks that can be switched on and off in your screenwriting software or template in less than a minute.
As usual, if you choose to ignore this advice but your story really is great - and your script manages to deliver it in an engaging way - then producers and reps may very well overlook these nitpicks no matter what any book or website tells you.